- explode: documentation ( source)
<?php
$str = "mx.google.com; dkim=pass header.i=@outlook.com header.s=selector1 header.b=OvtBcHsM; spf=pass (google.com: domain of xxxx@outlook.com designates 12.12.12.12 as permitted sender) smtp.mailfrom=xxxx@outlook.com; dmarc=pass (p=NONE sp=QUARANTINE dis=NONE) header.from=outlook.com";
$mail = explode(";",explode("smtp.mailfrom=", $str)[1])[0];
echo $mail;